A wireless power transmission or a wireless energy transfer refers to a technology for wirelessly transferring electric energy to desired devices. In the 1800's, an electric motor or a transformer employing the principle of electromagnetic induction has been extensively used and then a method for transmitting electrical energy by irradiating electromagnetic waves, such as radio waves or lasers, has been suggested. Actually, electrical toothbrushes or electrical razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction. Until now, the long-distance transmission using the magnetic induction, the resonance and the short-wavelength radio frequency has been used as the wireless energy transfer scheme.
In the case of a short-distance wireless power transmission, which has been spotlighted in these days, a wireless power transmitter is installed in a building in such a manner that a mobile device, such as a cellular phone or a notebook computer, can be continuously charged when a user uses the mobile device in the building even if the mobile device is not connected to an additional power cable.
However, in the above wireless power transmission technologies, a coupling co-efficient between a wireless power transmitter and a wireless power receiver must be equal to or higher than the critical value in order to facilitate the wireless power transmission using resonance. At this time, the coupling coefficient may be determined depending on the size of a transmission resonant coil of the transmitter and a reception resonant coil of the receiver and the distance between the transmitter and the receiver.
In general, the size of the reception resonant coil is significantly smaller than the size of the transmission resonant coil, so the coupling coefficient between the transmission resonant coil and the reception resonant coil is very small.
If the coupling coefficient between the transmitter and the receiver is lower than the critical value, magnetic induction may prevail over magnetic resonance required for the wireless power transmission. Thus, the effective wireless power transmission may not be achieved between the wireless power transmitter and the wireless power receiver.